Transparent resin films are used for automobile interiors, appliance exteriors, wallpapers and the like for purposes of decorating them with patterns such as a wood grain tone or the like; conferring designability such as a metallic tone or the like; and protecting (conferring scratch resistance and weather resistance on) base materials (for automobile interior, appliance exteriors, wallpapers and the like). Among these, acrylic resin films excellent in transparency, weather resistance, and surface hardness are widely used for decorating films having a function for protecting base materials.
However, a film consisting only of a methacrylic resin is very brittle, and difficult to be formed into a film, and further has very poor handling properties. Therefore, problems such as breakage may occur when trimming upon film-forming, when cutting the film after film-forming, when bonding the film to a base material, or when removing unwanted portions (burrs) after bonding the film to a base material.
Accordingly, in order to reduce the brittleness of the aforementioned film consisting only of a methacrylic resin, acrylic resin films in which so-called core-shell particles are blended have been suggested.
For example, Patent Document 1 suggests an acrylic resin film in which core-shell particles (commonly called two-layered core-shell particles) are blended into a methacrylic resin, the core-shell particles being obtained by co-polymerizing methacrylic acid alkyl ester and acrylic acid alkyl ester in the presence of cross-linked particles of acrylic acid alkyl ester polymer.
The acrylic resin film in which the above two-layered core-shell particles are blended usually has low surface hardness. Accordingly, in order to improve it, so-called three-layered core-shell particles (Patent Document 2) and an acrylic resin film in which the three-layered core-shell particles are blended have been suggested (Patent Documents 3 or 4). If the surface hardness of the acrylic resin film were high, the scratch resistance of the film would be improved, having an advantage as a protective film.
Meanwhile, core-shell particles themselves have no fluidity because they are cross-linked. Therefore, when core-shell particles are blended into a methacrylic resin, a portion of the core-shell particles inevitably protrude from a surface of a film formed by a film-forming process (for example, melt forming with a T die or inflation forming). This reduces the surface smoothness of the acrylic resin film.
In particular, three-layered core-shell particles generally do not easily undergo deformation as compared with two-layered core-shell particles. Therefore, an acrylic resin film comprising three-layered core-shell particles has a higher surface hardness, but a significantly lower surface smoothness as compared with an acrylic resin film comprising two-layered core-shell particles.
A decreased surface smoothness of a film tends to increase the brittleness of the film when cutting the film after film-forming, when bonding the film to abase material, or when removing unwanted portions (burrs) after bonding the film to a base material.
Further, when forming a layer comprising metal and/or metal oxide on a surface of a formed article, in particular a film of an acrylic resin, the surface smoothness of the formed article has very significant impacts. That is, the surface smoothness of a formed article directly affects the surface smoothness of a layer comprising metal and/or metal oxide formed on the surface of the formed article, which in turn affects the handling properties and designability such as mirror gloss of a laminated product.
Further, in a case where the surface of another resin is decorated with a formed article of an acrylic resin on which a layer comprising metal and/or metal oxide is formed by the simultaneous injection molding and in-mold laminating method, surface gloss is also significantly decreased because the surface smoothness of the formed article of the acrylic resin is low.
Further, another problem is that whitening occurs upon bending and heating when a film containing core-shell particles is subjected to secondary processing, resulting in deteriorated appearance.